Juan Huang

Summary Statement: A subset of monoterpenes provides a strong field signature of realised herbivory in tropical trees. This study highlights the value of integrating volatile traits into multi‑trait frameworks for assessing plant–insect interactions under field conditions.

Nitrogen (N) deposition disrupts mineral element dynamics, exacerbating phosphorus (P) limitation and inducing multiple nutrient imbalances. Although P addition is widely adopted to mitigate these negative effects by enhancing P availability, how multi-mineral elements in tropical trees respond to N and/or P addition remains poorly understood, particularly regarding their tissue-specific concentrations and inter-element relationships. Here, we investigated the effects of a decade-long N, and/or P addition on mineral element concentrations across tissues in two typical plantation tree species, Eucalyptus urophylla S. T. Blake (EU) and Acacia auriculiformis A. Cunn. ex Benth. (AA), in southern China. We also examined how these additions altered correlations among elements. Our results showed that both EU and AA maintained stable macro-element levels under long-term N addition, yet experienced significant changes in their micro-elements. This was evident by increased root aluminium (Al) and iron (Fe) concentrations in EU and decreased leaf Fe concentrations in AA. However, tissue-specific responses differed. EU exhibited significant response ratios in root mineral elements, whereas AA had negative response ratios in leaves and branches. Under long-term P and N + P addition, both species accumulated higher P and sodium (Na) but lower potassium (K), with significant response ratios in leaf mineral elements. Crucially, long-term N or/and P addition altered elemental correlation patterns. Specifically, long-term N addition strengthened sulfur (S) interaction with other elements in both species, whereas long-term P disengaged P from other elements in AA, and long-term N + P addition disrupted P interconnectedness in EU. Moreover, long-term N + P addition simplified mineral element network interactions in both species. These shifts in elemental correlations highlight potential cascading effects on ecosystem structure and function. Our findings demonstrate that tropical trees dynamically adjust mineral element concentrations across tissues and reconfigure inter-element relationships in response to N- and P-induced environmental changes. These adjustments have profound implications for nutrient cycling and ecosystem resilience in tropical forests under global changes.

Leguminous species have an advantage in acquiring phosphorus (P) compared with non-leguminous species. Nonetheless, it remains unclear whether this advantage would diminish under long-term nitrogen (N) deposition. In the seventh year of a simulated long-term N deposition experiment, we sampled surface soil to measure acid phosphatase activity (ACP) in stands dominated by leguminous and non-leguminous species, respectively. To assess the response of ACP to prolonged N addition, we also collected data on ACP in the second, eighth, twelfth, and thirteenth years of the experiment. We found that the difference in soil ACP between the two plantation types disappeared after long-term N input, and this process was accelerated under high N addition. This occurred due to an exacerbated P-limitation, which primarily prompts ACP production only in non-leguminous species. Additionally, there was a relatively decreased N contribution efficiency to ACP in the stand with leguminous species, indicating that soil N content no longer primarily governs ACP. This study demonstrates that prolonged high N deposition accelerates the loss of the “P-acquiring advantage” in leguminous plantations. To elucidate the differences in P usage strategies between leguminous and non-leguminous species under global change, more systematic research is warranted in the future.

Restoration plantations in subtropical regions, often established with fast-growing tree species such as Acacia auriculiformis A. Cunn. ex Benth and Eucalyptus urophylla S. T. Blake, are frequently developed on highly weathered soils characterized by phosphorus deficiency. To investigate strategies for mitigating nutrient imbalances in such ecosystems, a long-term (≥13 years) fertilization experiment was designed. The experiment involved three fertilization regimes: nitrogen fertilizer alone (N), phosphorus fertilizer alone (P), and a combination of nitrogen and phosphorus (NP) fertilizers. The objective of this study was to investigate the effects of long-term fertilization practices on soil quality in subtropical plantations using a soil quality index (SQI). Consequently, all conventional soil physical, chemical, and biological indicators associated with the SQI responses to long-term fertilization treatments were systematically evaluated, and a principal component analysis (PCA) was conducted, along with a literature review, to develop a minimum dataset (MDS) for calculating the SQI. Three physical indicators (silt, clay, and soil water content), three chemical indicators (soil organic carbon, inorganic nitrogen, and total phosphorus), and two biological indicators (microbial biomass carbon and phosphodiesterase enzyme activity) were finally chosen for the MDS from a total dataset (TDS) of eighteen soil indicators. This study shows that the MDS provided a strong representation of the TDS data (R² = 0.81), and the SQI was positively correlated with litter mass (R² = 0.37). An analysis of individual soil indicators in the MDS revealed that phosphorus addition through fertilization (P and NP treatments) significantly enhanced the soil phosphorus pool (64–101%) in the subtropical plantation ecosystem. Long-term fertilization did not significantly change the soil quality, as measured using the SQI, in either the Acacia auriculiformis (p = 0.25) or Eucalyptus urophylla (p = 0.45) plantation, and no significant differences were observed between the two plantation types. These findings suggest that the MDS can serve as a quantitative and effective tool for long-term soil quality monitoring during the process of forest sustainable management.

Restoration plantations in subtropical regions, often established with fast-growing tree species like Acacia auriculiformis and Eucalyptus urophylla, are frequently developed on highly weathered soils characterized by natural phosphorus (P) deficiencies. To investigate strategies for mitigating nutrient imbalances in such ecosystems, a long-term (≥ 13 years) fertilization experiment was designed. The experiment involved three fertilization regimes: nitrogen (N) fertilizer alone, P fertilizer alone and a combination of N and P (NP) fertilizer. The objective of this study was to investigate the effects of long-term fertilization practices on soil quality in subtropical plantations through a soil quality index (SQI). Principal component analysis (PCA) was used along with a literature review to select a minimum dataset (MDS) to calculate SQI. Three physical indicators (silt, clay, soil water content), three chemical indicators (soil organic carbon, inorganic nitrogen, total phosphorus), and two biological indicators (microbial biomass carbon, phosphodiesterase enzyme activity) were chosen as MDS from a total dataset (TDS) of eighteen soil indicators. Our study showed that long-term fertilization did not significantly change the soil quality as measured by SQI in both Acacia auriculiformis (p=0.25) and Eucalyptus urophylla (p=0.45) plantations. However, investigations of individual soil indicators in the MDS suggested that P addition through fertilization (P and NP regimes) strongly increases the P pool in the subtropical plantation soil. The MDS provided a strong representation of the TDS data (R2=0.81), and the SQI was positively correlated with littermass (R2=0.37). These findings suggest that the MDS can serve as a quantitative and effective tool for long-term soil quality monitoring.

Nitrogen (N) deposition alters the soil environment for forest trees, particularly in tropical regions, leading to variations in leaf traits. However, the adaptive responses of plantation tree species to chronic N deposition, via leaf trait modifications, remain poorly understood. We conducted a decade-long experiment involving N additions in two typical plantations dominated by Eucalyptus urophylla (EU) and Acacia auriculiformis (AA) in South China, to investigate species-specific leaf trait plasticity under N deposition. Our results showed that long-term N addition did not affect N and phosphorus (P) concentrations, sugar and starch levels, intrinsic water use efficiency (iWUE), and leaf mass per area (LMA), but lowered leaf total Carbon (C) content in both EU and AA. Moreover, it resulted in divergent traits between them, showing an increase in tannin and phenolics but a decrease in leaf water content (LWC) in AA but no such variations in EU. These differential responses were attributed to their unique leaf traits that EU contains high chemical defensive compounds and AA, as an N-fixing tree species, exhibits higher resource levels. The reduced leaf total C was redirected towards defense, without compromising iWUE through unchanged sugar and starch levels, particularly in AA. Our findings demonstrate that long-term N addition intensifies the coupling between C and water, resulting in a shift in C allocation in trees. Consequently, long-term N addition triggers different defensive strategies: a conservative defense in EU and an active defense in AA. This finding offers new insights into the adaptive mechanisms of forest plants under global change scenarios.

Soil acidification instead of alkalization has become a new environmental issue caused by urbanization. However, it remains unclear the characters and main contributors of this acidification. We investigated the effects of an urbanization gradient on soil acidity of Pinus massoniana forests in Pearl River Delta, South China. The soil pH of pine forests at 20-cm depth had significantly positive linear correlations with the distance from the urban core of Guangzhou. Soil pH reduced by 0.44 unit at the 0–10 cm layer in urbanized areas compared to that in non-urbanized areas. Nitrogen deposition, mean annual temperature and mean annual precipitation were key factors influencing soil acidification based on a principal component analysis. Nitrogen deposition showed significant linear relationships with soil pH at the 0–10 cm (for ammonium N ("Equation missing"-N), P < 0.05; for nitrate N ("Equation missing"-N), P < 0.01) and 10–20 cm (for "Equation missing"-N, P < 0.05) layers. However, there was no significant loss of exchangeable non-acidic cations along the urbanization gradient, instead their levels were higher in urban than in urban/suburban area at the 0–10 cm layer. Our results suggested N deposition particularly under the climate of high temperature and rainfall, greatly contributed to a significant soil acidification occurred in the urbanized environment.

Nowadays, nitrogen (N) deposition has become a growing global concern due to urbanization activities increasing the large amount of reactive N in the atmosphere. However, it remains unclear whether urbanization affects the composition and main sources of N deposition in rapidly urbanizing areas such as in China. One-year measurement of wet inorganic N deposition was conducted using ion-exchange resin (IER) columns in the range of 260 km from urban to rural areas in the Pearl River Delta (PRD) region, south China. An increasing pattern of wet inorganic deposition along the urbanization gradient was observed and it increased in the order: rural (15.26 ± 0.20 kg N ha⁻¹ year⁻¹) < suburban/rural (21.45 ± 3.73 kg N ha⁻¹ year⁻¹) < urban (31.16 ± 0.44 kg N ha⁻¹ year⁻¹) < urban/suburban sites (34.15 ± 5.73 kg N ha⁻¹ year⁻¹). Nitrate N (NO₃ ⁻–N) accounted for 53.5–79.1 % of total wet inorganic N deposition, indicating a significant negative correlation with distance from the urban core. Based on moss δ¹⁵N-values the main source of NO₃ ⁻–N was considered to be emitted from vehicles. Our results demonstrate that urbanization has large impacts on the regional pattern of wet inorganic N deposition. Thus, controlling NOx emission, especially vehicle emission will become an effective strategy for N pollution abatement in China.